Printed circuit boards (PCBs) are employed as a foundation for the mounting of various electronic components making up a circuit or system. Among the characteristics needed for good printed circuit board operation is the ability to electrically connect the various components efficiently and reliably. This is typically done through the use of etched patterns of metal conductors on the surface of the printed circuit boards.
However, the development of complex electronic systems has increased requirements for high packaging densities and high-speed wide bandwidth operation. High-speed logic families such as FAST, ECL, and GaAs (gallium arsenide) circuits have made the design of transmission lines on PCBs a critical factor for signal transition times below one nanosecond. PCB layout is now very critical to maintaining signal integrity, such as preserving signal edges and reducing distortion due to reflections and crosstalk. Proper impedance control and impedance termination are required in order to achieve desired signal fidelity.
Typical printed circuit boards are not suitable for such high frequencies due to excessive signal radiation from the conductive tracks. These conductive tracks, or signal traces, ultimately radiate more energy than they transmit when the frequency exceeds about 500 MHz or go high in the GHz band. Thus, there is a need to develop printed circuit boards which are compatible with such high frequencies.
Another type of equipment which has signal traces for transmission of signals at high frequencies are known as extender cards. These cards are designed to be connected to printed circuit boards and to carry all, or substantially all, of the signals from one printed circuit board to another. As with PCBs, it has been common to employ etched conductors on the surface of the extender cards as signal conductors.
Due to the large number of signals which sometimes have to be handled by an extender card, innovations in this technology have involved the use of signal carrier tape rather than printed circuit boards as the foundation for the conductors. One type of carrier tape is referred to as "flex tape", and is commercially available in many forms. In this approach, one or more layers of flexible tape are each provided with a pattern of conductive tracks. In applications where there is need for more tracks than will fit on a single layer of flex tape, the use of multiple layers has been adopted. This, however, has the disadvantage of requiring each layer to be independently etched, and the layers must then be properly combined in alignment so that the proper connections can be made to the conductive tracks. For high-speed systems, it is also important to control the impedance and provide proper isolation and shielding to preserve signal integrity. Current flex tape-based extender cards usually have an appreciable line length that has undesirable inductance for high-speed logic signals.